Work machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy-machinery use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to a pump on the work machine that provides pressurized fluid to chambers within the actuators. As the pressurized fluid moves into or through the chambers, the pressure of the fluid acts on hydraulic surfaces of the chambers to effect movement of the actuator and a connected work tool. When the pressurized fluid is drained from the chambers it is returned to a low pressure sump on the work machine.
One problem associated with this type of hydraulic arrangement involves efficiency. In particular, the fluid draining from the actuator chambers to the sump has a pressure greater than the pressure of the fluid already within the sump. As a result, the higher pressure fluid draining into the sump still contains some energy that is wasted upon entering the low pressure sump. This wasted energy reduces the efficiency of the hydraulic system.
One method of improving the efficiency of such a hydraulic system is described in U.S. Pat. No. 6,748,738 (the '738 patent) issued to Smith on Jun. 15, 2004. The '738 patent describes a hydraulic regeneration system having a first actuator, a second actuator, a third actuator, and a source of pressurized fluid. A directional control valve is disposed between the source and each of the first, second, and third actuators. An accumulator is used to store pressurized fluid and selectively discharge pressurized fluid to increase the efficiency of the work machine.
The system of the '738 patent is configured to regenerate hydraulic energy during operation under an overrunning load. In particular, when a load on an actuator naturally assists movement of the actuator in a desired direction, fluid exiting the actuator is pressurized by the load to a useful level. The system of the '738 patent directs this gravity-pressurized fluid from the actuator through the associated directional control valve to assist the source of pressurized fluid, to assist other actuators within the system, and to fill the accumulator. Once the accumulator is filled, the reserve of pressurized fluid therein is used to supplement or replace fluid typically provided by the source to the actuators, to provide torque-assist to the source, to assist propulsion of an associated work machine, and to torque-assist an associated engine by driving the source as a motor. During a regeneration event, the output of pressurized fluid from the source may be reduced or cease completely.
Although the system of the '738 patent may have improved efficiency compared to a conventional hydraulic system, it may be expensive and limited. Specifically, each of the three directional control valves includes a set of four independent metering valves. This large number of metering valves may significantly increase the cost of the system. In addition, because operation of the source varies in response to a regeneration event, operation of the engine driving the source may also vary. If the engine operation varies enough, efficiency of the engine may be reduced. Furthermore, the system of the '738 patent does not provide a way to utilize the source to power retract an actuator during a regeneration event associated with that actuator. Without this ability, power retraction of the actuator may be very inefficient.
The hydraulic regeneration system of the present invention solves one or more of the problems set forth above.